DMAX2DMAX
On my 6.2, I changed rear end, engine, transmission and greased the joints, all Delvac 1 and Mobil 1. I have fairly detailed fuel records for about 29K miles (13 months). ~18k miles before ~11k miles after. I was previously running Mystic 15-40 in the engine. Throwing out a few oddballs ,Speedo cable broke, trailer etc, I went from 17.1 average to 18.4 average. Most impressive were the highs going from 18s to 18.9. 19.9, 21, 21.4,22. {21 years old, 350,000 miles and still capable of 20-22mpg!}
I didn’t get as drastic results with my GMC Gasser with oil change only. I didn’t have as much of a baseline (~2k), but appears to have added .2-.4 mpg.
PM and I'll send you a spread sheet of the 6.2 results.